1. Field of the Invention
The present invention relates to an ultrasonic motor having high drive efficiency, which causes a stator to generate a traveling vibration wave, thereby driving a rotor. More particularly, the present invention relates to an ultrasonic motor having high drive efficiency, which causes an elastic member to generate a traveling vibration wave upon vibration having a vibration frequency in an ultrasonic range of a piezoelectric member, thereby driving a rotor.
2. Related Background Art
The structure of a conventional ultrasonic motor is disclosed in, e.g., JP-A-62-77068. FIG. 6 is a sectional view of the ultrasonic motor. A stator 1 is constituted by integrally fixing a piezoelectric member 1b to an elastic member 1a. A radial support member 3 is integrally molded on an inner peripheral surface of the elastic member 1a near a neutral axis of the stator 1 as a whole. The support member 3 has a thin plate portion 3a provided to the inner peripheral surface of the elastic member 1a, and a thick portion 3b provided on the peripheral edge of the thin plate portion 3a and having a larger thickness than that of the thin plate portion 3a. A technique for integrally molding the support member to the elastic member in this manner is also disclosed in, e.g., JP-A-59-213286. The thick portion 3b is fixed in a case 5 which receives a bearing 4.
A flange portion 6a radially projects from the inner periphery of the rotor 2, and a thick portion 6b is integrally molded on the peripheral edge of the flange portion 6a. A compression force by a given means (not shown) is applied to the thick portion 6b through a bearing 7, so that a lower surface 8 of the rotor 2 is urged against a drive surface 1c of the elastic member 1a.
When an AC voltage is applied to the piezoelectric member 1b, the piezoelectric member 1b causes bending vibration, and a traveling vibration wave is generated in the elastic member 1a. This vibration wave frictionally drives the rotor 2. The generation mechanism of the traveling vibration wave is described in detail in JP-A-60-245482 and a description thereof will be omitted.
In the above structure, when the thick portion 3b of the support member 3 is fixed by a pressing 40 and a fixing cylinder 50 or their equivalents shown in FIG. 2, the following problems are posed. If a lower end face 3c of the thick portion 3b of the support member 3 is not flat, when the thick portion 3b is fixed by the pressing 40 and the fixing cylinder 50 shown in FIG. 2, an internal stress is generated in the elastic member 1a, thus decreasing drive efficiency of the ultrasonic motor. On the other hand, if a junction surface 1d of the elastic member 1a contacting the piezoelectric member 1b is not flat, when the piezoelectric member 1b is fixed to the elastic member 1a, an internal stress is generated in the elastic member 1a, and also decreases drive efficiency of the ultrasonic motor. As shown in FIG. 6, when the lower end face 3c of the thick portion 3b and the junction surface 1d of the elastic member 1a do not exist on the same plane, it is almost impossible to simultaneously polish these surfaces.
The structure of a conventional stator is disclosed in, e.g., JP-A-60-245482. FIGS. 10 and 11 show this structure. A stator 10 is constituted by adhering an annular elastic member 11 and an annular piezoelectric member 12. A plurality of electrodes 13 (FIG. 11) are formed on the upper surface of the piezoelectric member 12, and electrodes 14a to 14d are also formed on its lower surface. The elastic member 11 is adhered to the piezoelectric member 12 so as to be electrically connected to all the electrodes 13. These electrodes are magnetized to alternately have opposite polarities. One-end portions of lead wires 15 and 16 are respectively soldered to the electrodes 14a and 14b. An AC voltage is applied to the electrodes 14a and 14b through these lead wires 15 and 16. The other end of a lead wire 17, one end of which is grounded, is soldered to the electrode 14c. The electrode 14c is electrically connected to the elastic member 11 through a conductive adhesive 18.
In the above-mentioned structure, all the electrodes 13 are grounded through the elastic member 11, the conductive adhesive 18, the electrode 14c, and the lead wire 17. Therefore, when an AC voltage is applied to the electrodes 14a and 14b, this is equivalent to apply a voltage to the widthwise direction of the piezoelectric member 12. When a predetermined AC voltage is applied to the electrode 14a through the lead wire 15, and an AC voltage having a 90.degree. phase difference from the predetermined voltage is applied to the electrode 14b through the lead wire 16, the piezoelectric member 12 is vibrated by these applied voltages. Upon this vibration, a traveling vibration wave is generated in a drive surface 11a of the electric member 11.
However, in this structure, the elastic member 11 and the electrode 14c are electrically connected through the conductive adhesive 18 to ground the electrodes 13. Therefore, the adhesive 18 is degraded by vibration of the elastic member 11 in resulting in a grounding error of the electrodes 13. Since ultrasonic vibration has a high frequency, it has a peeling effect as demonstrated in ultrasonic washing. Thus, the ground lead wire 17 may be directly connected to the elastic member 11 through a screw or the like so as not to disconnect the lead wire 17. However, undesirable vibration occurs in the connected portion due to the weight of the screw, and as a result, smooth driving is disturbed.
In some motors, as disclosed in Japanese Patent Laid-Open (Kokai) No. 59-178988, a comb-like groove is formed in the drive surface of the elastic member to improve drive efficiency.
As a support method of the stator, the following methods are known:
(1) A method of supporting the stator by arranging a shock absorber such as a felt on the lower surface of the piezoelectric member;
(2) A method of supporting the stator by arranging a flange-like support member extending from an outer or inner peripheral surface of the stator near a neutral surface and clamping the support member by fixing members; and
(3) A method of supporting the stator by arranging a plurality of rod-like sub vibration members radially extending from the outer or inner peripheral surface of the stator near a neutral surface, and placing the distal ends of the vibration members on the support member, as disclosed in Japanese Patent Laid-Open (Kokai) No. 60-96183 (corresponding to U.S. Pat. No. 4,634,915).
However, in the method (1) of supporting the stator by the felt, the felt has poor weather resistance, and a drive condition largely changes due to aging of the felt, resulting in poor reliability. In the method (2) of supporting the stator by the flange, since a bending strength of the flange portion cannot be lowered, drive efficiency is decreased. In addition, in the method (3) of supporting the stator by the rod-like sub vibration members, since a large number of rod-like sub vibration members having a small width and thickness must be arranged to have high dimensional precision, and are difficult to work, resulting an expensive structure. If each rod-like sub vibration member is thick, its mechanical strength is increased, and the vibration of the stator is externally transmitted through the support member, resulting in generation of noise.